The present invention relates to a system for controlling the clutch torque of an electromagnetic clutch disposed between an engine and a transmission of a motor vehicle, and more particularly to a system for controlling the variation of the clutch torque from a low clutch torque while an accelerator pedal is released to a high clutch torque upon depressing the pedal.
Japanese Patent Application Laid-open No. 57-30624 discloses a clutch control system in which clutch torque is maintained at a value larger than engine torque by a rated current as long as the vehicle is driven faster than a predetermined speed, irrespective of the condition of the accelerator pedal. In this system, when the accelerator pedal is released, irregular and abrupt fluctuation in engine torque occurs which is transmitted to a driving system, unpleasant shocking the driver. Further, because a large amount of rated current is needed to be supplied to the clutch under the lower engine speed condition while the accelerator pedal is being released, the electric power comsumption by a battery is great.
In order to remove such drawbacks, a control system was provided to absorb the torque variation. In the system, as shown in FIG. 4, the clutch torque is lowered to such a low value T.sub.1 as long as the engine braking can be effected while the accelerator pedal is released, although the clutch torque during the depression of accelerator pedal is fixed to a rated high value T.sub.2, as is in the conventional art.
On the other hand, when the accelerator pedal is depressed, the clutch torque rises from T.sub.1 to T.sub.2. However, since the rising speed of the clutch torque is higher than that of engine torque, the electromagnetic clutch fully engages before the engine torque reaches a sufficient value to drive the vehicle. Accordingly, engine speed can not be rapidly raised because of the sudden increase of load caused by rapid engagement of the clutch. As a result, vehicle speed decreases temporarily and stumbling of the vehicle occurs at the worst. In addition, the rapid engagement of the clutch causes a shock in the power transmission system.